                a. Field of the Invention        
The present invention pertains generally to digital-to-analog converters (DACs) and more particularly to two-stage non-buffered resistive string DACs.
b. Description of the Background
Two-stage resistor string DACs have been used to reduce the complexity of single-stage DACs including the number of resistors, the number of switches and the associated switching control circuitry. For example, single stage DACs for converting an N-bit digital signal require 2N resistors and 2N-1+2 switches. Active components such as unity gain operational amplifiers can be used between the two stages of a two-stage resistor string DAC to provide isolation between the resistor strings. More specifically, unity gain operational amplifiers have extremely high input impedance that provide isolation between the two stages of resistor strings. Although buffered two-stage resistor string DACs greatly reduce the complexity and the number of components required, the use of active components, as described above, requires that these circuits be active circuits, rather than simply passive circuits. Many present day applications require that circuits be passive and not consume power. For example, many battery-operated circuits, such as circuits in cell phones, PDAs, etc., must minimize the consumption of power. Elimination of the active buffers in a two-stage resistive string DAC degrades the performance of the DAC with respect to both the integral nonlinearity (INL) and differential nonlinearity (DNL).